Yes, I’m lucky that this service exists; I had nothing to do with creating it. I’m not any kind of insider, though; anybody near the city I live in (Philadelphia) can use it.
The service I use only accepts orders six days out. This was one of a number of smart changes they made when they got slammed by the pandemic. They used to allow orders two weeks in advance, which I imagine they did because some developer thought “what the hell, it’s possible”. When the pandemic hit, that of course meant everybody had to order two weeks out, because the excess demand guarantees slots will fill up the day they’re made available. So the store shrank the window to six days (which is still a long lead time to have to plan a grocery run).
They handle substitutions pretty well. An hour or so before my time slot, they text me a link to a form listing out-of-stock items and in some cases offering me a choice of one alternate pick or nothing. It would be better if they didn’t limit me to that alternate. (For example, my last order included Bumble Bee albacore packed in oil. that was out, so they offered me Bumble Bee albacore packed in water. A better substitute would have been another brand of albacore in oil. It would be nice if the alternate could be any item I name.) But what they have is better than nothing.